What is Sany SY245/265H Fault Code 104?

Fault Code 104 on the Sany SY245/265H excavator indicates a malfunction in the engine coolant temperature sensor circuit. This code is generated when the Electronic Control Module (ECM) detects an abnormal voltage signal from the coolant temperature sensor, suggesting either an out-of-range reading, open circuit, or short circuit condition.

The Mitsubishi Heavy Industries diesel engine powering these models relies heavily on accurate coolant temperature data for optimal fuel injection timing, idle speed control, and engine protection protocols. When Code 104 is active, the ECM typically defaults to a substitute temperature value, which can negatively impact engine performance, fuel efficiency, and potentially cause long-term damage if the engine overheats without proper monitoring.

Common Symptoms

• Check Engine Light or warning lamp illuminated on the instrument panel
• Engine running in limp mode or experiencing reduced power output
• Difficulty starting, especially during cold weather conditions
• Cooling fan running continuously at high speed regardless of actual engine temperature
• Increased fuel consumption and rough idle quality
• Potential engine overheating without corresponding dashboard warnings

Potential Causes

The most common technical failures triggering Code 104 on used Sany excavators include:

• Coolant temperature sensor failure due to age, corrosion, or internal element degradation
• Damaged or corroded wiring harness connections at the sensor plug—particularly common where harnesses contact the engine block or frame
• Chafed wiring insulation causing intermittent short circuits to ground, especially near mounting brackets
• Corroded or loose ground connections affecting sensor signal integrity
• ECM internal circuit failure (rare, but possible in high-hour machines)
• Contaminated coolant causing sensor element fouling or deposits
• Previous improper repairs using non-OEM sensors with incorrect resistance curves

How to Troubleshoot and Fix Code 104

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor and its wiring harness for obvious damage. On used excavators, check for harness wear at common rub points near the engine mount and valve cover. Look for corroded connectors, damaged insulation, or coolant leaks near the sensor location.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to measure resistance across the sensor terminals. At 68°F (20°C), resistance should typically read between 2,000-3,000 ohms (consult your service manual for exact specifications). Compare readings at different temperatures to verify the sensor's response curve.

Step 3: Circuit Verification With the sensor disconnected, check for voltage supply at the harness connector (typically 5V reference voltage from the ECM). Test for continuity between the sensor ground wire and chassis ground. Resistance should be less than 1 ohm.

Step 4: Signal Wire Testing Inspect the signal wire from sensor to ECM for shorts to ground or power. On used machines, pay special attention to areas where harnesses pass through metal grommets or contact hot engine surfaces.

Step 5: Component Replacement If sensor and wiring test within specifications, clear the code using Mitsubishi diagnostic software or compatible scan tool and monitor for recurrence. Replace faulty components with OEM-equivalent parts to ensure proper resistance values.

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Disclaimer: This guide provides general troubleshooting information. Always consult your equipment's service manual and consider professional diagnostic assistance for complex electrical issues or if you lack proper testing equipment.